The limit for total per-employer contributions in 2018 is $55,000. I have a 403b, a 457b, and a Defined Contribution plan through a single employer and I can make after-tax contributions to the DC plan. I'd like to do a mega backdoor roth via the DC plan's after-tax contributions.

If I contribute to the 403b, the 457b, and pre-tax to the DC plan, how much can I contribute after-tax to the DC plan? Do my contributions to the 403b and 457b count against the $55000 annual limit for this purpose?

my understanding is Yes. If you contribute $18k to 403, $18k to 457, then you can contribute only $19k ($55k-$18k-$18k) to your DC. I would not know how $19k would split in your DC since I do not know what are the rule for pre-tax contribution limits to your DC.

As far as I know, you should figure out whether you are allowed to do in-service distributions within your DC for after-tax money. If yes, then you can do Mega Back Door roth soon. If not, then you need to wait until you quit your employer and roll your money out of DC.

It is very cool that you have an opportunity to make after-tax non-Roth contributions. It is rare.

The 457b plan does not get taken into account when you determine what space is left in the $55,000 for your contribution maximum. The 457 is separate and it's own separate 55,000 contribution maximum limit, or 18,500 employee contribution limit.

FYI the Define Contribution plan in question is specifically a 401a plan. I called both my employer and the provider, Fidelity, and they said that the 401a has its own separate $55000 limit. This means that only my pre-tax contributions to the 401a count against the limit for the purposes of calculating how much I can contribute to the 401a post-tax. The 403b and 457b contributions do not count.